heat
Michele Claibourn, Chase Dawson, Carmen Wright
Heat and Heat Islands
Heat islands are areas with higher temperatures relative to outlying areas created by conditions like reduced vegetation, greater heat retention of impermeable surfaces like asphalt, paving, and cement, and heat generated by human activity.
In Summer 2021, an Equity Center team worked with (information about the summer program that generated the student images) to learn more about climate justice, including heat islands.
Heat islands have a range of impacts, including
- Increased energy costs for air conditioning
- Increased air pollutants and greenhouse gasses because of increased electricity use for cooling
- Increased harmful ground-level ozone from hot surfaces
- Reduced health and comfort
- Worse water quality and stress on warming aquatic systems
Source: EPA Heat Island Impacts
Heat Data
While students were (learning about climate and heat and evaluating their own environments ???), the Equity Center data team was working to generate more accessible heat data that could be integrated with information about people and landscapes.
Heat islands are most frequently analyzed using satellite imagery data, like the LandSat 8 imagery, used to estimate relative heat severity below.
Source: The Trust for Public Land and ESRI Living Atlas
To pair this kind of land surface temperature data with other variables, we looked at LandSat 8 imagery for a multiple days in the last few years that met a few conditions:
- the days were in the summer
- the images had less than X% cloud cover (which impedes temperature estimation)
- the images, or combinations of images, covered the majority of our local region
With the satellite images, we can extract temperature estimates for each 30-meter square area. We intersected the 30-meter squares with census block and census block groups – larger areas used by the census in estimating population characteristics – and calculated the minimum, maximum, median, and average value across every 30-meter square within a census block or block group.
Daytime land surface temperature measurements like these are typically higher than the air temperature felt by
Source: EPA, Learn about Heat Islands (https://www.epa.gov/heatislands/learn-about-heat-islands)
Heat in Charlottesville
Here we show the median land surface temperature within each Charlottesville city census block on July 4, 2020 as a deviation from the median recorded temperature in the city. The maximum air temperature recorded for Charlottesville on that day was 91 degrees Fahrenheit.
Hover over the map to see how much hotter or cooler a block was compared to the city as a whole.
Below you can compare the areas of greater heat to the areas with a higher percent tree canopy or a higher percent of surfaces covered with impervious materials.
Tree Canopy
The percent of an area covered with tree canopy is derived from the National Land Cover Database.
Impervious Surfaces
The percent of an area covered with impervious surfaces is derived from the National Land Cover Database.
Association with Land Surface Temp
The figures below plot the value of the tree canopy or the impervious surface estimates of each census block along the x-axis along the bottom and median temperature deviation from the overall land surface temperature of each census block along the y-axis on the side.
There is a very clear relationship – blocks with more tree canopy and less impervious surface have notably lower land surface temperatures.
Heat aross the Region
While heat islands are typically a feature of more urban settings, we used the same procedure to estimate the median land surface temperature throughout the Charlottesville region. In the figures below, we take the median temperature of each census block group – an area consisting of many census blocks – to make it easier to compare a broader region and to integrate information on population characterstics.
Land Surface Temperature, Trees and Surfaces
Tree Canopy
Impervious Surfaces
Association with Land Surface Temp
Land Surface Temperature, Populations
With land surface temperature aggregated to the block group, we can begin to visualize how temperature varies across the region as a function of the type of people who live there.
Income, Homeownership, Age
Charlotteville block groups evidence slightly higher temperatures on average, with the orange dots representing city areas hovering above zero. This an expected difference given the more urbanized and dense nature of the city.
Across the region, each of these population characteristics appears moderately related to land surface temperature differences, though the pattern for percent of residents who own their home versus rent is among the clearest: neighborhoods with higher rates of homeownership experience less heat.
Race and Ethnicity
The pattern of temperature by racial composition of a block group is weaker. There is not an obvious upward or downward sloping pattern to the scatter of points for the figures plotting land surface temperature by the percent of a neighborhood that is Black or Hispanic.
Youth Art
